We investigated the production of proinflammatory cytokines (IL-1 beta, IL-6, IL-8, and TNF-alpha) and immunoregulatory cytokines (IL-2, IFN-gamma, and IL-10) in the colonic mucosa of patients with active ulcerative colitis (UC), inactive UC, and non-inflammatory bowel disease (IBD) colitis by organ culture. The production of proinflammatory cytokines was significantly increased in all the studied groups compared with controls. In active UC, levels of these cytokines, except for IL-1 beta, were markedly increased compared with non-IBD colitis, and the levels were positively correlated with the degree of inflammation. Patients with non-refractory active UC receiving steroids showed levels of IL-1 beta and TNF-beta production similar to those in controls. IL-10 production was also significantly increased in all the studied groups, the value of being the highest in active UC. In contrast, IL-2- and IFN-gamma production was significantly decreased in both active and inactive UC compared with controls, and the values in active UC were inversely correlated with the degree of inflammation. In non-IBD colitis, decreased IL-2 production was observed, but IFN-gamma production did not differ from that in controls. In an experimental study, each of the proinflammatory cytokines was injected into the colonic mucosa of rats. All of these proinflammatory cytokines, except for IL-1 beta induced colonic mucosal damage that showed some histologic features similar to those of UC. These results suggest that the increased production of proinflammatory cytokines, particularly of IL-6 and IL-8, and the decreased production of IL-2- and IFN-gamma, probably downregulated by the enhanced production of IL-10, play an important role in the pathogenesis of UC.